A typical evaporative emission control system in a modern automotive vehicle comprises a vapor collection canister that collects volatile fuel vapors generated in the headspace of the fuel tank by the volatilization of liquid fuel in the tank. During conditions conducive to purging, the evaporative emission space which is cooperatively defined by the tank headspace and the canister is purged to the engine intake manifold by means of a canister purge system that comprises a canister purge solenoid valve connected between the canister and the engine intake manifold and operated by an engine management computer. The canister purge solenoid valve is opened by a signal from the engine management computer in an amount that allows the intake manifold vacuum to draw volatile vapors from the canister for entrainment with the combustible mixture passing into the engine's combustion chamber space at a rate consistent with engine operation to provide both acceptable vehicle driveability and an acceptable level of exhaust emissions.
Certain regulations require that certain future automotive vehicles powered by internal combustion engines which operate on volatile fuels such as gasoline have their evaporative emission control systems equipped with on-board diagnostic capability for determining if a leak is present in the evaporative emission space. It has heretofore been proposed to make such a determination by temporarily creating a pressure condition in the evaporative emission space which is substantially different from the ambient atmospheric pressure, and then watching for a change in that substantially different pressure which is indicative of a leak.
Commonly owned U.S. Pat. No. 5,146,902 "Positive Pressure Canister Purge System Integrity Confirmation" discloses a system and method for making such a determination by pressurizing the evaporative emission space by creating a certain positive pressure therein (relative to ambient atmospheric pressure) and then watching for a drop in that pressure indicative of a leak. Leak integrity confirmation by positive pressurization of the evaporative emission space offers certain benefits over leak integrity confirmation by negative pressurization, as mentioned in the referenced patent.
The invention of commonly owned U.S. Ser. No. 07/995,484, filed 23 Dec. 1992, and subsequently published as WO 94/15090 on 07 Jul. 1994, discloses means and method for measuring the effective orifice size of relatively small leakage from the evaporative emission space once the pressure has been brought substantially to a predetermined magnitude that is substantially different from ambient atmospheric pressure. Generally speaking, this involves the use of a reciprocating pump to create such pressure magnitude in the evaporative emission space and a switch that is responsive to reciprocation of the pump mechanism. More specifically, the pump comprises a movable wall that is reciprocated over a cycle which comprises an intake stroke and a compression stroke to create such pressure magnitude in the evaporative emission space. On an intake stroke, a charge of atmospheric air is drawn in an air pumping chamber space of the pump. On an ensuing compression stroke, the movable wall is urged by a mechanical spring to compress a charge of air so that a portion of the compressed air charge is forced into the evaporative emission space. On a following intake stroke, another charge of atmospheric air is created.
At the beginning of the integrity confirmation procedure, the pump reciprocates rapidly, seeking to build pressure toward a predetermined level. If a gross leak is present, the pump will be incapable of pressurizing the evaporative emission space to the predetermined level, and hence will keep reciprocating rapidly. Accordingly, continuing rapid reciprocation of the pump beyond a time by which the predetermined pressure should have been substantially reached will indicate the presence of a gross leak, and the evaporative emission control system may therefore be deemed to lack integrity.
The pressure which the pump strives to achieve is set essentially by its aforementioned mechanical spring. In the absence of a gross leak, the pressure will build toward the predetermined level, and the rate of reciprocation will correspondingly diminish. For a theoretical condition of zero leakage, the reciprocation will cease at a point where the spring is incapable of forcing any more air into the evaporative emission space.
Leaks smaller than a gross leak are detected in a manner that is capable of giving a measurement of the effective orifice size of leakage, and consequently the invention of the earlier application is capable of distinguishing between very small leakage which may be deemed acceptable and somewhat larger leakage which, although considered less than a gross leak, may nevertheless be deemed unacceptable. The ability to provide some measurement of the effective orifice size of leakage that is smaller than a gross leak, rather than just distinguishing between integrity and non-integrity, may be considered important for certain automotive vehicles.
The means for obtaining the measurement comprises a switch which, as an integral component of the pump, is disposed to sense reciprocation of the pump mechanism. Such a switch may be a reed switch, an optical switch, or a Hall sensor, for example. The switch is used both to cause the pump mechanism to reciprocate at the end of a compression stroke and as an indication of how fast air is being pumped into the evaporative emission space. Since the rate of pump reciprocation will begin to decrease as the pressure begins to build, detection of the rate of switch operation can be used in the first instance to determine whether or not a gross leak is present. As explained above, a gross leak is indicated by failure of the rate of switch operation to fall below a certain frequency within a certain amount of time. In the absence of a gross leak, the frequency of switch operation provides a measurement of leakage that can be used to distinguish between integrity and non-integrity of the evaporative emission space even though the leakage has already been determined to be less than a gross leak. Once the evaporative emission space pressure has built substantially to the predetermined pressure, the switch's indication of a pump reciprocation rate at less than a certain frequency will indicate integrity of the evaporative emission space while indication of a greater frequency will indicate non-integrity.